As well known, in high-speed components such as HEMTs (High Electron Mobility Transistors) which are used in satellite broadcasting receivers, high-speed logical circuits and power modules, a short gate length is required for a quick modulation while a large cross section of a pattern is also required in order to pass a high current therethrough.
From this, T-gates having a cross section of a ‘T’ shape are widely used in III-V compound semiconductor devices. The T-gate is formed of a gate head 102 and a gate foot 104 as shown in FIG. 1A, and has merits that it can maintain a gate resistance to be low while reducing the width of the gate foot 104.
To fabricate such T-gates with a gate foot width of 50 nm or below, an exposure process of the T-gate (i.e., electron beam lithography) and a process of stably forming the T-gate on a semiconductor substrate after depositing metals thereon are considerably important.
First, in the conventional exposure process of the T-gate having a gate foot width of 50 nm or below, the forward scattering and the gate head exposure have an effect on determining the gate foot definition. Here, increasing an accelerating voltage of an electron beam apparatus reduces the effect of the electron while decreasing sensitivity of the resist formed on the lowest layer reduces the effect of the gate head exposure. Further, the sensitivity of the resist can be reduced by developing it at a low temperature.
In order to fabricate the T-gate with a gate foot width of 50 nm or below by employing such an electron beam exposure process, the exposure process needs to be carried out by using an electron beam with an accelerating voltage of about 100 keV. Therefore, there are drawbacks in that its production cost is high and the semiconductor substrate can be damaged by the high accelerating voltage.
Next, the conventional T-gate having a straight gate foot is fabricated by decreasing the size of the gate foot to 50 nm or below in order to improve its characteristics, without decreasing the size of the gate head to prevent the increase of the gate resistance. However, since such size of the gate foot cannot support the gate head, the T-gate is not stably formed after depositing the metals and removing the resists, thereby resulting in a phenomenon where the T-gate falls to one side as shown in FIG. 1B.
If the T-gate formed in field effect transistors such as the HEMTs falls to one side, the schottky contact characteristics will be deteriorated, and thus, electrical characteristics of the semiconductor device will be degraded.
Accordingly, in the conventional forming process of the T-gate with a gate head having a wide width and a gate foot having a narrow width, the electron beam with a relatively high accelerating voltage is required to be used in the exposure process, and also a phenomenon that the T-gate falls to one side occurs due to the unstable structure of the T-gate. Therefore, there are problems in that the production cost is high and the characteristics of the semiconductor device are deteriorated.